Among the desirable properties of a photographic silver halide recording material is high sharpness. That is, the recording material should enable faithful reproduction and display of both coarse and fine details of the original scene. This combination of properties has proven difficult to achieve in practice.
A general description of the nature of this problem may be found in T. H. James, ed., "The Theory of the Photographic Process," Macmillan, New York, 1977 and in particular at Chapter 20 of this text, pages 578-591, entitled "Optical Properties of the Photographic Emulsion" by J. Gasper and J. J. DePalma.
One method of improving sharpness, disclosed at U.S. Pat. No. 4,312,941 and at U.S. Pat. No. 4,391,884, involves the incorporation of a spatially fixed absorber dye in a film layer between the exposing light source and a layer comprising a conventional grain light sensitive silver halide emulsion. In these disclosures, the absorber dye is held spatially fixed either by means of a ballast group or by means of a mordanting material incorporated at a specified position in the film structure. Use of this spatial arrangement of absorber dye and emulsion reduces front-surface halation effects.
U.S. Pat. No. 4,439,520, inter alia, discloses the utility of sensitized high aspect ratio silver halide emulsions for use in light sensitive materials and processes. These high aspect ratio silver halide emulsions, herein known as tabular grain emulsions, differ from convention grain emulsions in many characteristics. One differential characteristic is the relationship between the emulsion grain thickness and the emulsion grain equivalent circular diameter. Conventional grain emulsions tend to be isotropic in shape and, when incorporated in a film structure, tend to be randomly oriented within a particular layer. Tabular grain emulsions however, tend to be anisotropic in shape and, when incorporated in a film structure, tend to align such that their major axis parallels the plane of the film base. This degree of anisotropicity is know as the emulsion aspect ratio (AR), typically defined as the ratio of the emulsion grain equivalent circular diameter divided by the emulsion grain thickness. The ability to control emulsion grain thickness and alignment within a film structure can enable the realization of otherwise unattainable degrees of recording material performance.
The optical properties of photographic recording materials incorporating tabular grain emulsions are described in great detail at "Research Disclosure", #25330, May, 1985, as are methodologies of specifying particular arrangements of tabular grain emulsions within a film structure and of specifying particular tabular grain emulsion thicknesses so as to enable the attainment of specifically desired properties, such as speed or sharpness in underlying or overlying emulsion layers. No mention is made of the relationship between tabular grain emulsion thickness and the speed or sharpness of the emulsion layer comprising such a grain.
These methods may not prove to be wholly satisfactory. U.S. Pat. No. 4,740,454, for example, discloses that although high frequency sharpness may be attained by the appropriate choice of tabular grain emulsion thickness and placement, this can be at the cost of low frequency sharpness. The term high frequency sharpness generally relates to the appearance of fine detail in a scene reproduction while the term low frequency sharpness generally relates to the appearance of clarity or "snap" in a scene reproduction. It is now understood that the terms "high frequency sharpness" and "low frequency sharpness" are qualitative in nature and that both image frequency, expressed as cycles/mm in the film plane, and the image magnification employed in producing a reproduction must be taken into account when specifying such terms. This publication discloses that both high frequency and low frequency sharpness may be simultaneously improved by the incorporation of specific mercaptothiadiazole compounds in combination with tabular grain silver halide emulsions. This practice may not be wholly satisfactory since the incorporation of such silver ion ligands can lead to deleterious effects on film speed and film keeping properties.
In a related area, U.S. Pat. Nos. 4,746,600 and 4,855,220 disclose that unexpectedly large degrees of sharpness can be attained by combining spatially fixed absorber dyes and Development Inhibitor Releasing Compounds (DIR Compounds) in a photographic silver halide recording material. The spatially fixed absorber dyes are positioned between an emulsion containing layer and the exposing light source. The materials described in these disclosures incorporate either conventional grain silver halide emulsions or low aspect ratio tabular grain silver halide emulsions. There is no indication of any dependence in film imaging performance on the thickness or spatial positioning of the light sensitive silver halide emulsion grains in these publications.
Again, in a related area, U.S. Pat. No. 4,833,069 discloses that large degrees of sharpness can be attained by simultaneously controlling imaging layer thickness to between 5 and 18 microns and incorporating large quantities, between 15 and 80 mol % of colored cyan dye-forming couplers, known also in the art as cyan dye-forming color masking couplers. This method may not be wholly satisfactory since the use of excessive quantities of color masking couplers can lead to inferior color rendition by over-correcting the color reproduction through excessive use of the masking function. Again, there is no indication of any dependence in film imaging performance on the thickness or spatial positioning of the light sensitive silver halide emulsion grains in this publication.
In yet another related area, U.S. Pat. No. 4,956,269 discloses that color reversal silver halide photographic materials incorporating tabular grain silver halide emulsions can show improved sharpness when the photographic layer incorporating the tabular grain silver halide emulsion also incorporates a quantity of absorber dye sufficient to reduce the speed of that layer by at least 20%, when the total imaging layer thickness is less than 16 microns and when the swell ratio of the film is greater than 1.25. The materials described in this disclosure incorporate intermediate aspect ratio (AR&lt;9.0) tabular grain silver halide emulsions. These conditions and constraints are non-predictive of the performance of color negative silver halide photographic materials.
A color negative silver halide photographic recording material incorporating conventional (non-tabular) grain silver halide emulsions and a quantity of distributed dye sufficient to reduce the speed of a color record by about 50% has been commercially available for many years. Additionally, it has been common practice in the photographic art to commercially provide silver halide photographic recording materials incorporating conventional grain and/or tabular grain silver halide emulsions in combination with soluble dyes sufficient to reduce the speed of a color record by about 10% for purposes related to ease of manufacture. Likewise, color negative silver halide photographic materials incorporating high aspect ratio tabular grain silver halide emulsions with an average grain thickness of circa 0.11 and 0.14 microns in an intermediately positioned layer have been commercially available for many years.
The thicknesses of the silver halide emulsions employed in this invention are adjusted for the purposes of improving film performance according to principles described in Research Disclosure, May, 1985, Item 25330. This disclosure teaches, by extrapolation from the optical properties of silver bromide sheet crystals, that the thicknesses of silver halide emulsions incorporated in specific photographic layers and sensitized to one spectral region may be chosen to enable either improved speed or improved sharpness behavior in other photographic layers incorporating silver halide emulsions sensitized to different regions of the spectrum. These improvements are said to occur because the light transmission and reflection properties of the silver halide emulsions are controlled in large part by their grain thicknesses. Further discussion on the relationship between the thickness of silver halide crystals and their reflectance properties can be found in Optics, by J. M. Klein, John Wiley & Sons, New York, 1960, pages 582 to 585. These disclosures make no teaching about the relationship between the thickness of a silver halide emulsion sensitized to a particular region of the spectrum and the sharpness behavior of a photographic layer or element using such an emulsion.
Despite all of this effort, fully adequate degrees of sharpness have not been attained in silver halide photographic materials comprising high aspect ratio tabular grain emulsions.